In a hot day or in the dessert, because the temperature at different height are not the same.
So the air density is not the same which cause the refraction index of light to be different at different height.
If it cause light to bent, then you will see strange image.
[img]http://www.phy.ntnu.edu.tw/demolab/phpBB/pics/241_reflects.jpg[/img]
Light emitted from the car move down then bent upward, and enter our eye.
It will appear to us that the light is coming from below the ground.
(Similar to a mirror was placed on the ground)!